Two columns of Russian tanks and military vehicles fired Grad missiles at a border post in southeastern Ukraine, then rolled into the country Thursday as Ukraine's overmatched border guards fled, a top Ukrainian official said.

The comments by Col. Andriy Lysenko, a spokesman for Ukraine's National Security Council, and other statements from NATO, the pro-Russian separatists in Ukraine and the United States left no doubt that the Russian military had invaded southeastern Ukraine.

A top NATO official said at least 1,000 Russian troops have poured into Ukraine with sophisticated equipment and have been in direct "contact" with Ukrainian soldiers, resulting in casualties. He called that a conservative estimate and said another 20,000 Russian troops were right over the Russian border.

"Russian forces have entered Ukraine," Ukraine's president declared Thursday, cancelling a foreign trip and calling an emergency meeting of his security council.

President Petro Poroshenko summoned the council as the strategic southeastern town of Novoazovsk appeared firmly under the control of separatists and their Russian backers, a new front in the war in eastern Ukraine between the separatists and Poroshenko's government in Kyiv.

"Today the president's place is in Kyiv," Poroshenko said.

Lysenko said the missiles from Russia were fired about 11 a.m. and about an hour and a half later, two columns, including tanks and other fighting vehicles began an attack. They entered Ukraine from Veselo-Voznesenka and Maximovo of the Rostov region in Russia.

Russian stock markets dived as fears grew that the country was escalating its role in the conflict, a move that could provoke the U.S. and European Union to impose further sanctions on Russian businesses and individuals. Russia's MICEX index dropped nearly 2 per cent on Thursday, and major Russian state banks VTB and Sberbank dropped more than 4 per cent.

Brig. Gen. Nico Tak told reporters at NATO headquarters that the ultimate aim of Russia was to stave off defeat for the separatists and turn eastern Ukraine into a "frozen conflict" that would destabilize the country "indefinitely."

"Over the past two weeks we have noted a significant escalation in both the level and sophistication of Russia's military interference in Ukraine," Tak said in Casteau, Brussels. "Russia is reinforcing and resupplying separatist forces in a blatant attempt to change the momentum of the fighting, which is currently favouring the Ukrainian military."

NATO also produced satellite images to provide what it called "additional evidence that Russian combat soldiers, equipped with sophisticated heavy weaponry, are operating inside Ukraine's sovereign territory."

Tak said the satellite images were only "the tip of the iceberg in terms of the overall scope of Russian troop and weapons movements."

"We have also detected large quantities of advanced weapons, including air defence systems, artillery, tanks, and armoured personnel carriers being transferred to separatist forces in Eastern Ukraine," he said. "The presence of these weapons along with substantial numbers of Russian combat troops inside Ukraine make the situation increasingly grave."

The leader of the insurgency, Alexander Zakharchenko, said in an interview on Russian state television that 3,000 to 4,000 Russians have fought on the separatist side since the armed conflict began in April.

The U.S. government accused Russia of orchestrating a new military campaign in Ukraine, helping rebel forces expand their fight and sending in tanks, rocket launchers and armoured vehicles.

On Thursday morning, an Associated Press journalist saw rebel checkpoints at the outskirts of Novoazovsk and was told he could not enter. One of the rebels said there was no fighting in the town.

Novoazovsk, which lies along the road connecting Russia to the Russia-annexed Crimean Peninsula, had come under shelling for three days, with the rebels entering it on Wednesday. This area had previously escaped the fighting that has engulfed areas to the north, and the only way rebels could have reached the southeast was coming through Russia.

The new southeastern front raised fears that the separatists are seeking to create a land link between Russia and Crimea. If successful, it could give them or Russia control over the entire Sea of Azov and the gas and mineral riches that energy experts believe it contains. Ukraine already lost roughly half its coastline, several major ports and significant Black Sea mineral rights in March when Russia annexed Crimea.

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